In modern waste water and sewage treatment systems, the effluent is often collected in large settling basins, which may be several hundred or more feet long. A chain and flight system within the settling basin is continuously driven so as to prevent the accretion of floating and settled layers, and to introduce controlled-flow patterns, to facilitate water purification in known fashion. The systems generally take the form of a common drive at one corner of the settling basin, engaged to one or more separate drive shafts and associated drive sprockets at each side of the basin. The drive sprockets run continuous chains that extend lengthwise along the top of the settling basin and return along the bottom of the settling basin in the opposite direction. To control and synchronize the chains, idler sprockets are mounted on transverse shafts that are supported rotatably in the side walls. Transverse collector bars or flights coupled to the chain extend across substantially the entire width of the settling basin. These flights move with the chains to direct floating matter toward one end of the basin, usually to a skimmer system at the downstream upper end of the basin, while on the lower return path settled matter is scraped upwardly and in the opposite direction. Because of the size of the system, the forces involved are substantial, in terms of the driving torque required, the reactive force exerted against the flights by the liquid as it is agitated, and the size and masses of the sprockets, shafts, and chains.
Since the clarifier system for the settling basin must operate virtually continuously, and because of the corrosive nature of the waste water and sewage effluents, significant demands are placed on these systems, these demands often being of a conflicting nature. For example, the materials used must be corrosion resistant and substantially unaffected by reactions with the effluent. The elements and subsystems must have long life, under conditions of virtually continuous use, which in turn means that they must also be very wear resistant. This, however, is a function not only of the materials used but of the forces exerted. It is evident that a lightweight, low mass, system involves lower friction and lower tensile stresses on the chain and sprockets, and therefore should result in a reduction in power demands. However, the lengths and sizes of the settling basins are such that the pulling forces on the chains can range from several thousand pounds to much more. Thus the chain system, at its weakest point, must have high tensile strength. The transverse flights must have adequate rigidity under the reactive loads that are imposed, since a common problem is that these flights can distort by bowing or twisting, causing misalignment of the chains and possible catastrophic failure.
Earlier chain systems were constructed of selected heavy metals treated for corrosion resistance. Flights were constructed of a variety of woods and likewise treated chemically for longer life spans. Such components, being massively built, require high power inputs to be driven. The industry more recently has turned to non-metallic systems, using machined, cast or molded sprockets and also fiber-reinforced chain links. Here, some practical limitations based upon materials and design considerations have been encountered. For example, to operate a chain and flight system for a long life span (e.g., 20 years) it is necessary to use a low wear, low fatigue type of construction. While many synthetic resins have low friction characteristics, they are not readily wear resistant, and also tend to creep, swell or elongate because of saturation effects from the liquid. Excessive elongation in a long chain loop over a period of time cannot be tolerated because of the dangers of catastrophic failure and binding in the sprocket drive system. In addition, even though the mass of a largely or entirely non-metallic system may be lowered in comparison to a system using metal components, the reactive forces of the effluent on the transverse flights, together with the mass of the units themselves, still impose substantial loads on the chain links. The response, in some instances, has been to utilize conventional filament winding techniques for making the chain links, so as to achieve the improved strength characteristics of filament winding utilized originally in tubular structures and pressure cylinders. There are limits on the degree of improvement that can be obtained with this technique, and furthermore there are economic limitations that arise from the fact that filament winding is a time-consuming and laborious process, even if automated.
Other load and wear factors introduce further complexities into system considerations. For example, "wear shoes" are commonly disposed on the flights to engage support rails extending along the upper sweep and lower return paths. Thus the flight and chains are externally supported during travel along the continuous loop in different regions. This adds substantial continuous frictional force that must be overcome by the driving forces. Wear on the sprockets and the engaging bushings or pins in the chain links should be substantially uniform, so that there is no point of incipient failure. The presence of grit or other particulate matter in the settling basin cannot be avoided, but should neither cause undue friction nor an increase in wear. The chain link should be stable, and not subject to elongation or creep that would introduce undue wear or create the danger of catastrophic failure. In addition to all these factors, if maintenance is necessary, parts replacement and assembly must be accomplished with a minimum of effort.